Slide-bead coating is known in the art for supplying a flowing liquid layer or plurality of liquid layers down a slide surface to an efflux end, or lip, at which a liquid bridge, or bead, is formed in the gap between the lip and a moving substrate. The moving substrate carries away liquid from the liquid inventory in the bead in the same layered structure as that established on the slide. Exemplary examples are described in U.S. Pat. Nos. 2,761,791 and 2,761,419 issued to Russell et al.
Initiation of the slide-bead coating process is customarily accomplished in the following sequence, shown in FIGS. 1 and 2. In FIG. 1, the flow of coating solutions 1 and 2 is initially established with a coating roll 7 and coating plate 3 far enough apart so as to allow the coating solutions 1 and 2 to flow as a moving film of liquid over the face of the coating plate 3 and into a chamber 14 from which it is drained through a tube 16 into a sump 17. The coating plate 3 with associated assembly and coating roll 7 are then brought into close proximity to establish a flow across a coating gap 5 between the coating plate 3 and a substrate 6 supported by the coating roll 7, as shown in FIG. 2. Typically, the start-up coating is thicker than the steady-state coating that follows due to a brief shearing flow transient occurring upon initial dynamic wetting of the substrate. Several problems occur as a result of the initial thick flow, such as streaks, material loss and the like. Dynamic wetting is typically not identical across the substrate, thereby causing an uneven start of the coating process. This generates material loss due to uncoated sections of the substrate, as shown in FIG. 3, wherein A represents unsuitable material which occurs prior to establishment of a steady state, and B represents the desired steady-state coating. In severe cases, dynamic wetting does not occur at all and steady-state coating is not established. This typically occurs with viscosities that are inappropriate, coating rates that are inappropriate, and the like, as known in the art. Furthermore, bubbles, gel particles and other materials tend to generate streaks which often continue well into the steady-state coating.
U.S. Pat. No. 3,220,877 discloses a method using higher-than-normal differential pressure for inducing high air-flow rates down across the coater face as the coating roll is moved near the coating plate. The beneficial result is a reduction in the excess coating thickness at coating starts. Other technologies claiming this result include U.S. Pat. No. 4,808,444 issued to Yamazaki et.al, a method and apparatus for rapidly moving the coating roll between positions of coating and non-coating, and U.S. Pat. No. 4,877,639 issued to Willemsens et.al., a method consisting of at least two distinct liquid layers with different viscosities and with different flow rates. These inventions may have a beneficial effect in reducing the excess coating thickness at coating start. However, the coating start can also detrimentally affect the uniformity of the coating even after the normal coating thickness is established.
Continuous coating typically requires a transition from the lag end of a first substrate to the lead end of a second substrate in sequence. To decrease the amount of time required to change substrates, it is desirable to physically connect the lag end of the first substrate to the lead end of the second substrate to form a continuous moving substrate. The physical connection is generally done, as known in the art, by a splicing tape preferably overlapping both the lag and lead ends of the corresponding substrates with a spliced seam. As the spliced seam transits through the liquid bridge, the steady state flow characteristics are disturbed, causing defects in the coating. Flow disturbances can themselves reach a steady state which causes defects, such as streaks, well into the steady-state portion of the coating. Methods to eliminate disturbances from a spliced seam include rapidly reversing the aforementioned initiation process just prior to the splice, and reinitiating the process just after the splice. Such a method is inferior for the same reasons mentioned above for coating initiation.
Methods for improving the coatability of splices have been advanced by U.S. Pat. No. 4,172,001, issued to Heetderks, which teaches a two-piece splice tape. Disturbances still occur with the two-piece splice tape, albeit at a lower frequency, and the operation of splicing is complicated by an additional step. U.S. Pat. No. 4,024,302, issued to Takagi, et.al., teaches projecting discontinuous areas on the second substrate, which requires an additional step for preparation of the substrate surface and further complicates the manufacturing operation.
Yet another problem in coating continuous substrates is the presence of a foreign particle or aberration on the surface of the substrate. As these aberrations pass through the coating gap, disturbances occur in a manner analogous to a splice. These disturbances are typically sporadic and unexpected, which severely complicates efforts to prevent their occurrence. The present invention provides a single method for decreasing the material loss due to coating initiation, substrate splices and substrate surface aberrations.